The invention relates generally to optical parametric oscillators (OPOs) and more particularly to a continuously tunable OPO operated in the near infrared range.
Optical parametric oscillators have been recognized as useful to effect the efficient conversion of fixed wavelength pump laser radiation into broadly wavelength tunable radiation. Thus, OPOs can provide an efficient source of high power coherent radiation at wavelengths which are not covered well by conventional lasers.
The applications of OPOs are numerous and include spectroscopy, environmental monitoring, remote sensing, chemical process control, and so forth. OPOs which operate in the ultraviolet (UV), visible and near infrared (IR) ranges are described in a number of publications. However, analytical and remote sensing applications, and others, would benefit greatly from the ability to extend the operating spectral range of OPOs to the mid-IR range, i.e., from roughly 3 .mu.m to about 10-15 .mu.m, because this range contains characteristic rotational-vibrational absorption bands of a large number of molecules, as well as "transparency windows" of the atmosphere.
Unfortunately, the operation of currently existing OPOs in this mid-IR range is limited by several factors, such as: (i) the unavailability of nonlinear crystals with sufficient transparency and birefringence at longer wavelengths; and (ii) the unavailability or poor performance (most notably, a low optical damage threshold) of existing optical elements such as mirrors, beamsplitters, etalons and so forth, designed for this spectral range.
Several groups have reported laboratory versions of OPOs and the contents of the following publications are incorporated herein by reference: (1) Knights, M. G., et al. "Multiwatt mid-IR optical parametric oscillator using ZnGeP.sub.2 ", Advanced Solid State Laser Conference (Salt Lake City, Utah, 1994); (2) Vodopyanov, K. L., et al., "Extrawide tuning range IR optical parametric generators", Conf. on Lasers and Electro-optics, 1996 OSA Technical Digest, pp. 334-344, (Optical Society of America, Washington, D.C. 1996); (3) Fan, Y. X., et al. "AgGaS.sub.2 infrared parametric oscillator", Appl. Phys. Letts., v.45, #4, 1984, pp. 313-315; (4) Elsaesser, T., et al. "Parametric generation of tunable picosecond pulses in the medium infrared using AgGaS.sub.2 crystals", Appl. Phys. Letts., v. 44, #4, 1984, pp. 383-385; (5) Cheung, E. C., et al. "Silver Thiogallate, singly resonant optical parametric oscillator pumped by a continuous-wave mode-locked Nd:YAG laser", Optics Letters, v.19, #9, 1994, pp. 631-633; (6) Eckardt, R. C., et al. Broadly tunable infrared parametric oscillator using AgGaSe.sub.2 ", Appl. Phys. Letts., v. 49, #11, 1986, pp. 608-610; (7) Budni, P. A, et al., "Kilohertz AgGaSe.sub.2 optical parametric oscillator pumped at 2 .mu.m, Optics Letts., v.18, #13, 1993, pp. 1068-1070; (8) Grasser, C., "ontinuous-wave mode-locked operation of a picosecond AgGaSe.sub.2 optical parametric oscillator in the mid infrared", Advanced Solid State Lasers, Technical Digest, OSA, 1996, pp. WD4-1-WD4-3; (9) Born, M., Wolf, E., "Principles of Optics" (Pergamon Press, Oxford, 1968). However, the performance of these laboratory OPOs suffers from problems relating to insufficient damage resistance and inefficient optical elements.
Parametric conversion is a second order nonlinear process and therefore, conversion efficiency and oscillation threshold depend on the intensity of the pump and oscillating beams. Therefore, the maximum achievable intensity is limited in many situations by the onset of damage to the optical elements.
The low threshold to damage of the optical elements employed in the foregoing laboratory OPOs in the mid-IR range makes it difficult to achieve sufficiently narrow spectral line width of the generated beams. This is, in part, because line narrowing elements in the OPO cavity introduce significant energy loss. This energy loss is then compensated for by increasing the pump intensity. OPOs reported in the foregoing references 1-8 produce relatively broad band output radiation, typically significantly wider than 10 cm.sup.-1 which is insufficient for many applications, such as to resolve the rotational-vibrational spectra of molecular gases.
The design of optical elements formed with multi-layer thin film coatings suitable for the mid-IR range presents several significant technological challenges. For example, many optical materials commonly used in UV, visible and near-IR ranges (such as Si, Hf, Ti, and Zr oxides) exhibit strong absorption beyond 3 .mu.m. Also, commonly used optical materials which are transparent, such as ThF.sub.4 and ZnSe, are insufficiently damage resistant. In addition, multilayered thin film coatings tend to be porous as an inherent result of the deposition process and therefore readily accumulate moisture, which becomes trapped at the surface irregularities and inside the thin films. Consequently, the strong absorption of water in the mid-IR range can cause a significant decrease of the laser damage threshold and in many instances, prevent the proper operation of the OPO.
Some precautions can be taken to reduce the water content in the coatings to provide significant improvements in the resistance of the optical elements to laser damage. These include operating the OPO in a dry atmosphere and/or keeping critical optical elements at a slightly elevated temperature, to drive off absorbed water. However, these measures increase the complexity of the device and make operating the device more cumbersome. Moreover, even these measures do not sufficiently improve the results for thick multi-layered coatings, which tend to have a more developed columnar structure.
U.S. Pat. No. 5,033,057, the contents of which are incorporated herein by reference, describes a cavity with separate dichroic pump steering mirrors, which couple the pump beam in-and-out of the cavity. Nevertheless, these dichroic steering mirrors require multi-layer thin film dielectric coating processes. This presents drawbacks, such as those described above, and limits the performance of the OPO. In addition, the input steering mirror is described as being positioned at the location of the highest intensity in the OPO cavity, because it is exposed to the incoming pump radiation as well as to the generated signal and idler beams.
Accordingly, it is desirable to provide an improved laser device including an optical parametric oscillator, capable of operating efficiently in the mid-IR range.